1. Field of the Invention
The present invention relates to a fuel assembly for a small-sized nuclear reactor using a coolant such as a liquid metal, and particularly, to a fuel assembly including a plurality of grids provided to a fuel bundle.
2. Related Art
Generally, in the nuclear reactor of the type mentioned above, a fuel assembly is supported in a reactor core while being attached to a support member. In a nuclear reactor using a coolant such as a liquid metal, the coolant is circulated around a plurality of fuel pins included in the fuel assembly supported in the reactor core. In this case, if the nuclear reactor is small-sized, the fuel assembly is configured to store the fuel pins in a wrapper tube to enable the circulation of the coolant with no need for a drive source. The wrapper tube includes an entrance nozzle at a lower end thereof for introducing the coolant, and an operation handling head at an upper end thereof.
The wrapper tube includes therein grids for supporting the fuel pins in the radial direction of the wrapper tube, and liner tubes inserted in the wrapper tube for fixedly holding the respective grids in the axial direction of the wrapper tube. The intervals in the radial direction of the fuel pins are kept by the grids. Meanwhile, the intervals in the axial direction of the grids are kept by a tie rod, the liner tubes, or the like (see Japanese Unexamined Patent Application Publication No. HEI 6-174882, for example).
With reference to FIGS. 8 to 11, a conventional example of a fuel assembly 100 will be described. In FIGS. 8 and 9, a plurality of fuel pins 101 are incorporated in a wrapper tube 103, with the pin intervals in the radial direction of the fuel pins 101 kept by grids 102. Each of the fuel pins 101 is fixed at a lower portion thereof by a lower pin support plate 105 and at an upper portion thereof by an upper pin support plate 106. From a coolant inlet 108 of an entrance nozzle 104 provided at a lower position, the coolant such as a liquid metal flows in and moves upward. Then, the coolant flows out from a coolant outlet 109 of a handling head 107.
In the thus configured fuel assembly 100, ring-shaped grids are used as the grids 102 having a low pressure drop. Further, as illustrated in FIG. 10, liner tubes 110 each formed by a thin hexagonal tube are provided on the inner surface side of the wrapper tube 103, i.e., outside a fuel bundle such that the liner tubes 110 and the grids 102 are alternately stacked. Thereby, the intervals in the axial direction of the grids 102 are kept.
Since the flow passage area around the fuel bundle is large, the cladding temperature of the fuel in a central area of the fuel bundle becomes relatively high. Therefore, there arises a need to keep the cladding temperature equal to or lower than a cladding temperature limit, and thus the thermal efficiency is decreased. To suppress this phenomenon, as illustrated in FIG. 11, there has been known a technique of providing the liner tubes 110 with peripheral flow preventing projections, which are formed by bending peripheral walls of the liner tubes 110.
As described above, there has been proposed in the conventional fuel assembly to provide the liner tubes with the peripheral flow preventing projections formed by bending the peripheral walls of the liner tubes, for example. According to the proposal, however, it is not necessarily easy to sufficiently suppress the peripheral flow.
To suppress the peripheral flow in the fuel assembly as much as possible, it is necessary to reduce the flow passage area formed between the wrapper tube and peripherally disposed ones of the fuel pins to be approximately equal to the flow passage area surrounded by other ones of the fuel pins disposed toward the center from the peripherally disposed ones of the fuel pins. Specifically, it is necessary to secure the flow passage area formed between the wrapper tube and the peripherally disposed fuel pins to be approximately equal to the flow passage area surrounded by other ones of the fuel pins disposed in a triangular array inside the peripherally disposed ones of the fuel pins.